1. Field of the Invention
The present invention relates to a liquid crystal display device having a transparent protection plate, and more particularly, to a liquid crystal display device having a protection plate and exhibiting improvement in brightness, visibility and viewing angle characteristic.
2. Description of the Related Art
A pen entry technology, which allows easy manipulations as if one is writing on a paper with a pencil, has become important owing to a trend toward diversification of information equipment and reduction in size and weight of portable terminals, and therefore, display devices having a touch panel which realizes both inputting and displaying are in a popular use. While various types of touch panels such as an optical type, an ultrasound type, a resistive membrane type, an electrostatic capacitance type, an electromagnetic induction type and the like are presently put to practical use, a transparent protection plate is usually used at a top surface of a touch panel in any type for the purpose of detecting an input signal or protecting a viewing screen. That is, a transparent protection plate is disposed to a top surface of a touch panel, or a transparent protection plate itself forms the touch panel. In addition, a liquid crystal display, which is thin in thickness and light in weight and consumes little electricity, is in a frequent use for the purpose of displaying. Especially a ferroelectric liquid crystal display has an orientation defect when subjected to external shock, a transparent protection plate which is disposed in front of the ferroelectric liquid crystal display serves as protection of a liquid crystal layer against the external shock.
In such a liquid crystal display device having a transparent protection plate, since there is reflection from the transparent protection plate in addition to surface reflection from the liquid crystal display, a displayed view shows extremely poor in a bright room or in the open air. As a solution of this visibility problem, Japanese Patent Application Laid-Open No. 5-127822 proposes to use a circular polarizing plate comprising a xcex/4-plate and a polarizing plate. Meanwhile, Japanese Patent Application Laid-Open No. 10-48625 proposes a method for improving a display quality, in which another xcex/4-plate is disposed between a liquid crystal display device and a touch panel having a circular polarizing plate. The later publication describes that the xcex/4-plate which forms the circular polarizing plate and the another xcex/4-plate are arranged so that the directions of orientation axes (optical axes) of the two xcex/4-plates are in the same direction, or perpendicular to each other, and the perpendicular-direction arrangement is preferable because of better visibility.
However, such a structure that the xcex/4-plate which forms the circular polarizing plate and the another xcex/4-plate are arranged so as to have the direction of their orientation axes arranged perpendicular to each other in order to cancel a phase retardation has been found to have a problem that a displayed view looks yellow when viewed from an oblique direction. In the meantime, when the two xcex/4-plates are arranged so that the direction of their orientation axes are parallel to each other and the two xcex/4-plates function as a xcex/2-plate, the visibility deteriorates as described in the publication above.
An object of the present invention is to provide a liquid crystal display device having a protection plate and exhibiting improvement in brightness, visibility and viewing angle characteristic.
As a result of dedicated efforts to solve these problems, the present inventor discovered that it is possible to improve coloring of a displayed view in an oblique direction while maintaining a display quality in a front direction by means of such combination of a xcex/4-plate and a phase retarder that the sum (xcex1A+xcex1B) of the wavelength dispersion (xcex1A) of the xcex/4-plate, which forms a circular polarizing plate, and the wavelength dispersion (xcex1B) of a phase retarder, which is disposed between a transparent protection film and a liquid crystal display, is in the range of 1.11 to 1.95, thereby arriving at the present invention.
That is, the present invention provides a liquid crystal display device comprising:
a laminated protection panel composed of a xcex/4-plate, a polarizing plate and a transparent protection plate;
a liquid crystal display; and
a phase retarder;
wherein the laminated protection panel is disposed to a top surface of a liquid crystal display with a distance, the phase retarder is arranged between the laminated protection panel and the liquid crystal display, and the sum (xcex1A+xcex1B) of a wavelength dispersion (xcex1A) of the xcex/4-plate and a wavelength dispersion (xcex1B) of the phase retarder is in the range of 1.11 to 1.95.